1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method.
2. Description of the Related Art
In a lithography process to manufacture a device, such as a semiconductor device, an exposure apparatus is used to transfer a pattern on a reticle (original) onto a wafer (substrate) coated with a resist. As the density and degree of integration of integrated circuits increase, the exposure apparatus requires a higher resolution, and, accordingly, the wavelength of the exposure light becomes shorter. More specifically, as a light source for the exposure apparatus, a KrF excimer laser (wavelength: 248 nm) and an ArF excimer laser (193 nm) have been introduced to replace mercury lamps.
An increase in the numerical aperture (NA) of the projection optics of the exposure apparatus can also increase the resolution. Hence, an immersion exposure apparatus is also proposed in which a liquid with a refractive index larger than one fills the space between the projection optics and the wafer, to increase the numerical aperture (NA).
As the resolution of the exposure apparatus increases, strict requirements are imposed on aberration measurement and its correction in the projection optics of the exposure apparatus. For example, it is required to accurately identify and to adjust the influence of a slight change in lens position, which occurs when transporting the exposure apparatus, and changes in optical characteristics caused by heat during an exposure process. Therefore, after mounting the projection optics on an exposure apparatus main body, it is indispensable to measure the optical characteristics of the projection optics in the exposure apparatus and to adjust them to be optimal. The optical characteristics do not depend on polarization, to be described later, but are identical to conventional wavefront aberration, and will, accordingly, be explained as non-polarization aberration hereafter.
As the numerical aperture (NA) increases, the adverse influence of polarization becomes significant. Studies have been made on a method of controlling polarization of the exposure light. However, even by controlling the polarization of the exposure light by using an illumination system to achieve a desired polarization state, the polarization state in the projection optics can change due to distortion caused by holding a lens or by heating of the lens, use of a birefringent material, such as fluorite, and the like. This leads to an increasing demand to accurately identify a change in polarization in the projection optics. Optical characteristics resulting from birefringence of the projection optics depend on polarization of an incident beam and will, accordingly, be explained as polarization aberration hereafter. Polarization aberration includes retardation caused by birefringence, and its azimuth. Other than retardation and azimuth, polarization aberration may be expressed in various manners, e.g., a Stokes parameter, a Jones matrix, and a Mueller matrix.
In this manner, as the performance of the exposure apparatus improves, demands increase to measure, as the optical characteristics of the projection optics, non-polarization aberration, which does not depend on polarization and polarization aberration which does, quickly and accurately.
As a method of measuring the non-polarization aberration and polarization aberration of an optical system, Japanese Patent Laid-Open No. 2002-71515, No. 2004-257854, and No. 2-116732 propose the use of an interferometer dedicated to measurement. PCT Publication No. WO 2003/028073 proposes the use of an aberration measurement unit to measure non-polarization aberration and a polarization converting unit to measure polarization aberration. Japanese Patent Laid-Open No. 2005-116732 proposes a Mueller-matrix measurement method for an exposure apparatus.
However, the above methods include various problems. Japanese Patent Laid-Open No. 2002-71515, No. 2004-257854, and No. 2-116732 propose interferometers dedicated to measurement, which are difficult to apply to measurement in an exposure apparatus main body. This is because any one of the above interferometers is configured to measure the wavefront of a detection target surface by referring to a reference surface, as in a Mach-Zehnder interferometer. More specifically, with the above methods, the optical path is long and complicated, and not a common path, so the adverse influence of vibration is non-negligible. In addition, generally, an exposure light source has a very short coherence length and is, accordingly, unsuited for measurement in an exposure apparatus. Moreover, when mounting an interferometer in an exposure apparatus, an image sensing device, serving as a detector, must be arranged on the stage. Hence, weight factors are also significant.
Namely, to mount an interferometer in the exposure apparatus, the interferometer must enable measurement with lenses, and be resistant to vibration, be compact, light-weight, and accurate. When measurement is to be performed using exposure light from the exposure light source, the interferometer must also be capable of measurement using light with a short coherence length.
Specific problems will be described hereafter. Japanese Patent Laid-Open Nos. 2002-71515 and 2004-257854 propose separate calculations of wavefronts, which depend on polarization, and wavefronts which do not, from the measurement results of two perpendicular polarized light beams. However, from measurement of two perpendicular incident polarized light beams, only a change in wavefront, which depends on polarization, is obtained. It is impossible to measure retardation and azimuth, which are birefringence information of the optical system, simultaneously and accurately. As retardation and azimuth of birefringence must be obtained accurately as aberration caused by polarization, the methods described above are not sufficient for this. Since many of the methods require measurement light having a long coherence length, exposure light cannot be used as measurement light.
Japanese Patent Laid-Open No. 2-116732 uses a Mach-Zehnder interferometer, in which the beam is parallel light. As the optical path is branched to cause interference between a reference wavefront and a detection surface, the interferometer is sensitive to vibration, requiring a long coherence length. It is, therefore, very difficult to form such an interferometer arrangement in the exposure apparatus. Except for that, Japanese Patent Laid-Open No. 2-116732 discloses nothing concerning the interferometer. In addition, at least four linearly polarized light beams are necessary.
PCT Publication No. WO 2003/028073 discloses the use of two linearly polarized light beams for aberration measurement. As described above, with only two linearly polarized light beams, the retardation and azimuth of birefringence cannot be calculated accurately. PCT Publication No. WO 2003/028073 also discloses a method of separately measuring only birefringence by a combination of a wavelength plate and a polarizer. In this case, although the retardation and azimuth of birefringence can be calculated, wavefront aberration cannot be calculated. Hence, non-polarization aberration and polarization aberration must be calculated using separate units. This increases the unit driving time and the measurement time, making it impossible to perform high-speed measurement. The two units must be aligned with the optical axis. Depending on the alignment accuracy, a difference may occur in measurement of the aberration by two of the units. This can increase the error.
With the method of Japanese Patent Laid-Open No. 2005-116732, although the Mueller matrix can be calculated accurately as polarization aberration, non-polarization aberration must be measured separately. Japanese Patent Laid-Open No. 2005-116732 does not disclose a method for this separate measurement.
The object of each of the prior art documents as described above is different from that of the present invention, which is aimed at simultaneous measurement of non-polarization aberration and polarization aberration.
Furthermore, each of Japanese Patent Laid-Open No. 2002-71515, No. 2-116732 and No. 2005-116732, and PCT Publication No. WO 2003/028073 has the premise that measurement is performed with parallel light. This is for the following reason. In Japanese Patent Laid Open No. 2002-71515, and No. 2005-116732, and in PCT Publication No. WO 2003/028073, a phase shifter, a polarizer, and the like, are arranged before a light-receiving portion. As the phase shifter and polarizer have characteristics that differ depending on the incident angle, they can be used only for measurement with parallel light. Likewise, since Japanese Patent Laid-Open No. 2-116732 uses a Mach-Zehnder interferometer, it has a premise that measurement is performed with parallel light. Therefore, the methods of Japanese Patent Laid-Open No. 2002-71515, No. 2-116732 and No. 2005-116732, and PCT Publication No. WO 2003/028073 can be adapted only for measurement with parallel light.
As described above, the conventional methods cannot measure non-polarization aberration and polarization aberration (retardation and azimuth of birefringence) as the optical characteristics of the projection optics, simultaneously, at high speed and accurately, in the exposure apparatus.